javascript运算符“in”

Question

I'm used to python, so

a = [1,2,3]
1 in a # -> True
b = ["1", "2", "3", "x"]
"x" in b # -> True

Why is it that in JavaScript

a = [1,2,3]
1 in a // -> true
b = ["1", "2", "3", "x"]
"x" in b // -> false

and much weirder

"1" in b // -> true

Answer 1

in works on KEYS of arrays, not values. 1 in a succeeds because there is an element #1 in your array, which is actually the 2 value.

"1" fails, because there is no 1 PROPERTY or KEY in your array.

Details here: https://developer.mozilla.org/en/JavaScript/Reference/Operators/in

Answer 2

The thing you have to understand about JavaScript is almost everything is an "Object" that can have properties. Array's are just a special type of object whose properties are integer indexes and have push, pop, shift, unshift, etc. methods. Plus they can be defined with the square bracket shorthand you used:

a = [1,2,3];

This creates an Array object with the properties:

a[0] = 1;
a[1] = 2;
a[2] = 3;

Now as others have said, all the in operator does is check that an object has a property of that name and a[1] == 2 therefore 1 in a == true . On the other hand,

b = ["1", "2", "3", "x"];

created an Array object with the properties:

b[0] = "1";
b[1] = "2";
b[2] = "3";
b[3] = "x";

So b["x"] == undefined therefore "x" in b == false .

The other thing you have to understand is JavaScript uses "duck typing", meaning if it looks like a number, JavaScript treats it like a number. In this case, b["1"] == 2 therefore "1" in b == true . I'm not 100% certain whether this is duck typing at work or JavaScript just always treats property names as Strings.

If you wanted to declare a generic object that wouldn't have the Array methods but had the same properties you would write:

var b = {"0": "1", "1": "2", "2": "3", "3": "x"};

Which is shorthand for:

var b = {}; // This declares an Object
b[0] = "1";
b[1] = "2";
b[2] = "3";
b[3] = "x";

Answer 3

Because "x" in b is looking for a property name 'x' . There is none, they are all numerical (but still string) property names, considering it is an array.

Answer 4

1 is a valid vanilla numeric array index, x is not; in works with array indexes/object member names, not their values.

Answer 5

“1”不是数组对象的属性...